Large containers have become common place for the bulk transport of goods. Conventional containers are typically hollow metal rectangular prisms with a pair of doors opening one end. When not in use the containers are stored in multiple stacks. These stacks may be as many as eight containers high.
In order to stack the containers they must be lifted by a forklift to the top of the stack. Specialised forklift attachments have been developed for this task. Most of these attachments grab the containers at coupling pieces provided at the corners.
A variety of different mechanisms have been devised for grabbing the containers. Many of the prior art mechanisms lift single containers only. One example is described in U.S. Pat. No. 5,382,067 which discloses a side latch assembly that grabs a container from above. The side latch assembly incorporates a hook that engages the coupling piece at the corner of the container.
Other container lifting assemblies utilise twist lock mechanisms to engage the corner pieces of the container. An example of this type of container handling assembly is found in the specification of Australian patent application number 71330/91.
Prior art mechanisms that lift only a single empty container at a time can cause unnecessary delays in loading and unloading operations. In order to speed up operations it is desirable to lift and stack two containers at a time. For many prior art assemblies this is not possible because the latch mechanism extends above the container. Lifting of two containers is only possible with those mechanisms that access the coupling pieces from the front of the container. However, the use of these mechanisms is dangerous since the upper of the two containers is not held by the mechanism. The upper container can easily shift during movement, at best resulting in an uneven stack, and at worst resulting in damage or injury.
The safety problem has been partly overcome by lift assemblies that incorporate a pair of latch assemblies to engage the coupling pieces of both the upper and lower containers. Such an assembly is described in European patent application number 0638512 in the name of Mandigers BV. This patent application describes a carry device for picking up two containers placed one on top of the other. Coupling means are arranged in pairs such that a coupling means is inserted to the coupling piece of each container. The pair of coupling means are pivotally connected to a frame of the carrying device.
The Mandigers assembly has a number of shortcomings. It requires an hydraulic actuating piston to operate the upper coupling means thereby requiring a supply of hydraulic pressure to be supplied into the lifting head This can limit the maximum height extent of the lifting mechanism. Furthermore, as the height to which the containers are lifted is increased the positioning of the coupling means into the coupling piece becomes very difficult. With the Mandigers assembly it is possible for the containers to move with respect to each other and to become dislodged from the coupling means.